1. Field of the Invention
The present invention relates generally to the field of telephone communications. More specifically, the present invention discloses a repeater unit for accessing the data link provided in T1 spans supporting the ESF format.
2. Statement of the Problem
T1 spans have been widely used in this country for many years to provide digital telephone communications between channel banks. When a T1 line is used as an interoffice transmission link the operating telephone companies have access to both ends of the line for maintenance and performance monitoring. Furthermore, most installations include protection switching facilities that can also be used to routinely test a line while the traffic assigned to that line is diverted to a spare line. A problem arises when a T1 line is used by a customer as access to the public switched network, as a leased line in a private network, or to interconnect between two adjacent operating telephone companies. The recent rapid growth in use of T1 lines in such applications has created the need for enhanced maintenance and performance monitoring features for T1 customers. The extended superframe (ESF) format as provided in the D5 channel bank (produced by AT&T) addresses many of these features.
Customers should be able to nonintrusively monitor end-to-end performance of the T1 circuit. Monitoring bipolar violations at the customer service unit (CSU) provides nonintrusive monitoring of the received signal, but provides no information regarding the quality of the signal received at the other end of the line. Furthermore, leased T1 circuits generally involve intermediate transmission, multiplexing, and cross-connect equipment. Because the interfaces of this equipment remove bipolar violations, the customer has no performance statistics internal to the provider's facilities. Determination of end-to-end error rate performance in this environment requires taking at least a portion of the T1 circuit out of service.
ESF allows customers to achieve end-to-end performance monitoring while the link is in service by redefining the 8-kbps F bit in each frame to consist of a 2-kbps framing channel, a 4-kbps data link channel, and a 2-kbps CRC (cyclic redundancy check) channel representing a check sum over all information bits. The ESF format is shown in greater detail in FIGS. 5 and 6. The data link supports numerous services, one of which is the ability to interrogate the far end, or any intermediate equipment, for performance statistics. Thus, the data link provides both the customer and the provider with a very useful tool for isolating faulty span lines or equipment. As shown in FIG. 5, the three subchannels are established by "extending" the superframe format to encompass 24 DS1 frames. Because the actual framing bits occur only once every four frames, there are 771 bits intervening between bits carrying the frame pattern sequence (001011). The six CRC bits of each extended superframe provide a cyclic redundancy check of all 4608 information bits in the previous superframe. Besides providing end-to-end performance monitoring, the CRC virtually precludes the chances of false framing on a data bit position. Even though static user data can possibly simulate the framing pattern sequence, it is extremely unlikely that user data can spuriously generate valid CRC codes in successive superframes.
The performance parameters typically measured and reported by the 4-kbps data link are framing bit errors, CRC errors, severely errored framing (SEF) events, line code violations, and controlled slip events. Individual events are reported as well as event summaries. Examples of performance summaries reported are:
(a) Errored seconds (ES). PA1 (b) Severely errored seconds (SES.fwdarw.319 ES's). PA1 (c) Controlled slip seconds PA1 (d) Unavailable seconds.
Customer service units supporting ESF typically determine the above parameters on 15 minute intervals and store them for up to 24 hours for polling by a controller. In addition to supporting remote interrogation of performance statistics, the data link can be used to carry alarm information, loopback commands, and protection switching commands.
There is presently a need for a unit that can be readily inserted into a T1 span to access the ESF data link for performance monitoring and communications. In addition, ESF has not been universally adopted in the telecommunications industry. As a result, there are many situations in which it is necessary to provide an ESF/SF interface where a channel bank at one end of the T1 span supports ESF and the channel bank at the other end does not. This can occur between adjacent telephone operating companies or over a leased T1 circuit between a customer service unit supporting ESF and a channel bank that does not support ESF. In such cases, there is a need for an ESF/SF interface unit that can be inserted in the T1 span to provide performance monitoring for that portion of the T1 span between the interface unit and the channel bank that is ESF compatible.
3. Solution to the Problem
The present invention addresses these shortcomings by providing a drop-in unit that can be inserted into a T1 span to access the ESF data link either for the purpose of performance monitoring or communications in addition, the present invention can serve as an ESF/SF interface where the channel bank at one end of the T1 span supports ESF and the channel bank at the other end does not.